The present invention relates in general to collision warning and avoidance systems for motor vehicles, and, more specifically, to an adjustable threshold for initiating driver assistance actions in response to predicted collisions.
Several different types of driver assistance systems have been developed that involve the remote sensing and tracking of other vehicles or objects that may approach or impact a host vehicle. Scanning radar systems, camera-based optical based recognition systems, or combinations of the two are typically used to find relevant objects, determine their relative speed and travel path, and predict whether and when an impact may take place if the vehicles remain on their current trajectories. The driver assistance system may simply warn a driver as in a forward collision warning (FCW) system, thereby allowing the driver to determine and execute any necessary evasive maneuver. Other types of driver assistance systems such as an adaptive cruise control (ACC) system may automatically initiate an evasive action wherein a set cruising speed is reduced when a lead vehicle is detected, or such as a brake-steer system which may apply unequal braking forces to different wheels in order to steer the host vehicle away from a potential collision.
Initiation of the driver assistance action (whether it is the issuance of a warning or an automatic intervention to avoid a predicted impact) may or may not be trigger depending on how imminent a potential impact actually is. In a typical FCW system, a time-to-impact threshold has been used in order to decide whether a potential impact is sufficiently imminent to justify a warning. The threshold is based on the current host vehicle speed and the relative closing velocity of the target on the collision course. The threshold is typically defined in terms of the estimated amount of time that would elapse until the predicted impact. The amount of time for the threshold may typically be longer when the host vehicle travels at higher speeds. Using the predetermined time threshold and the relative closing velocity of the target, a warning alert is generated when the target approaches the host vehicle within a distance equal to the relative velocity divided by the threshold.
The length of the time-to-impact threshold is generally determined by a vehicle manufacturer in advance such that it provides sufficient opportunity for the driver to take any necessary evasive action, while at the same time ensuring that unnecessary warnings are not generated. The selection of a time-to-impact threshold depends on various parameters and driving conditions which assume certain nominal performance characteristics of the host vehicle such as braking and steering performance, vehicle weight, and others. The threshold settings also are based on assumptions regarding typical road conditions, weather, and other environmental variables. The resulting thresholds typically provide good overall performance for most situations. However, vehicle parameters change with aging of a vehicle, and weather or roadway conditions may frequently change in ways that have an effect on the time it takes to effectively execute an avoidance maneuver. For instance, variable road surface conditions resulting from precipitation, temperature, a rough surface, or sand or gravel coating the road can increase the time required in order to slow down or steer a vehicle. Various factors such as tire wear can also affect surface friction and the ability to generate a torque needed to slow or steer the vehicle. Thus, a situation in which a two second time-to-impact threshold would be appropriate under normal conditions may be insufficient when experiencing particularly unfavorable conditions. It would be desirable to increase the time-to-impact threshold to better match the actual conditions influencing the maneuverability of the host vehicle.